View Full Version : DPI and Megabytes how to calculate

19-May-2004, 09:19
Is the a simple formula to comcert the DPI specs of a scanner to calculate how large a scanned file will be in Megabytes??

Also a way to calculate file sizes for 300 - 360 dpi printing of 24x 30 inch prints


tim atherton
19-May-2004, 09:37
Do you have photoshop?

the quickest/laziest way is just open up a file (or make a blank file) then go into resize and just input the numbers - it will show you the resulting size. You don't actually have to make the file bigger - just use the dialogue box as a "calculator"

Amadou Diallo
19-May-2004, 10:23
A widely accepted formula: File Size = (height x width x bit depth x dpi2) / 8

*Note "dpi2" means dpi squared.* For size in MB carry decimal over 6 places from the final number. This gives a slightly higher number than the Photoshop dialog box, (computer HD bit storage?) but you'll get an idea.

Peter Witkop
19-May-2004, 10:29
Well to make a 24x30 at 300 ppi, take 24" x 300 = 7200, then 30x300 = 9000, so your file will be 7200 x 9000 pixels. To figure the file size, multiply 7200 x 9000 = 64800000, which is the total number of pixels. Now the file size is also determined by what type of file it is, RGB, Grayscale, or CMYK, and the number of bits per chanel. Working with 8 bit chanels, with a grayscale image, there is 1 byte per pixel, so multiply the number of pixels by 1 for the total number of bytes in the file, multiply by 3 for the number of bytes in an RGB image, or 4 for a CMYK image. To bring those numbers into units that are more easily understood, divide the file size in bytes by 1024 to get the size in kilobytes, then by 1024 again for the size in megabytes (note 1024, not 1000 since we're ultimatly dealing with binary information, so it has to be a power of 2). So for our example, we'd get a 61.8 megabyte file for grayscale, for RGB, a little over 185 megabyte file. If you were using 16 bit chanels, you'd multiply the number of pixels by 2 for each chanel (8 bits per byte), so you'd multiply by 2 for grayscale, 6 for RBG, etc. To figure the file size of a scan, you'd do it the same way, say for a 4x5 scan, at 1200 ppi, multiply 4 and 5 by 1200 to get the dimension of the file in pixels, then figure the file size the same I did above. Also as was mentioned above, photoshop makes a handy way to calculate things quickly. Hope that helps.


Amadou Diallo
19-May-2004, 10:45
Correction: >>For size in MB carry decimal over 6 places from the final number.<<

Should be: For kilobytes divide by 1024. For MB divide again by 1024. This will match the Photoshop file size. (Thanks Peter, I forgot about the binary conversions).

Leonard Evens
19-May-2004, 12:58
The short answer is no.

You can calculate the number of pixels by multiplying each linear dimension of the source in inches by the DPI figure and then multiplying both these numbers together.

After you know the number of pixels, you can then multiply by either 3 for 24 bit color depth or 6 for 48 bit color depth. That will give you the number bytes needed to store the image, unless compressed. There may be a few additional bytes for information about the storage format. To get the number of megabytes from that, divide by 1024. But all that is for uncompressed formats. Even lossless formats like tiff are usually compressed in some way. That will reduce the size of the file by some factor depending on the format, the compression algorithm, and the image.